Thickness control systems for rolling mills



July 9, 1963 c. A. vossBERG THICKNESS CONTROL SYSTEMS FOR ROLLING MILLS Filed Dec. 5. 1960 mm2; IZO

United States Patent O 3,096,671 THICKNESS CONTROL SYSTEMS FOR ROLLING MILLS Carl A. Vossberg, Umatilla, Fla. Filed Dec. 5, 1960, Ser. No. 73,753 9 Claims. (Cl. iso-56) This invention relates to rolling mills and, more particularly, to yautomatic thickness controls applicable to rolling mills in general, which mills areused for thickness reduction of strip material.

Automatic controls for this purpose are not broadly new, but prior art attempts at effective thickness control have not been entirely commercially successful. A conventional roll stand will include, in addition to the rolls themselves and `a prime mover for driving them, a screwdown motor which is effective to change the pressure between the rolls. The pressure between the rolls must necessarily be changed to take care of such factors as `different materials, different widths of materials and different thickness differential between the up and downstream side of the mill. For example, a particularly hard material will require greater movement of the screwdown motor to produce an equivalent thickness change than would be the case for a softer material. Further, an automatic thickness control of a conventional type which is set to provide a given thickness change for a given material, will not produce an equivalent thickness change for a different material. 'Ihe settings of such controls for the screwdown movement, when set for a hard material, would cause overcorrecting or hunting of the output thickness for a softer material. Manual methods for setting this control factor have not been satisfactory because of the inherent human error which tends to provide either incorrect adjustments or insufficient control in an attempt to avoid hunting.

The present invention has, for its principal object, an automatic control system for eliminating the human error in setting the control factor. Other objects of the present invention may be listed as follows:

(l) To provide an electrical automatic thickness control system which is consistently effective in producing a pre-determined thickness reduction regardless of variations in material, composition or material dimensions.

(2) To provide in an automatic thickness control system a means for disabling the automatic control feature whenever the speed of processing is too low or the mill stopped.

Other objects of the invention will be apparent to those skilled in the art from the following detailed description read in conjunction with the attached sheet of drawings, in which:

FIGURE l is a greatly simplified lschematic block diagram of -an automatic control arrangement; and

FIGURE 2 is a more detailed, but nevertheless, schematic of the control system itself.

In general, the foregoing objects of the present invention are achieved by energizing the screwdown motor basically in response to deviations in stock thickness as sensed by, for example, an X-ray thickness gauge. This basic control function, however, is modified by controlling the time of energization of the screwdown motor in response to several different factors, including the magnitude of thickness variation, as well as the load on the mill and the speed of the mill.

Referring to FIGURE 1 of the attached drawing, a strip or work piece 1 is shown as traveling from left to right in the figure as it passes through the roll stand having a pair of rolls 2 and 3. While the drive means or prime mover for the roll stand is not shown, it will be understood that the rolls 2 and 3 are positively driven in the direction in- 3,096,67 1 liatentetjl `Iuly 9, 1963 c CC `the roll stand involved. The apparatus may also include unwind and windup rolls as shown in the drawing at 6 and 7, and the strip may be under tension during the time it is undergoing thickness reduction in passing through the rolls 2 and 3. Positioned on the downstream side of the roll stand is a thickness gauge which is identified by the reference number 8 and which could conveniently be of the non-contacting typesuch, for example, as an X-ray thickness gauge. The gauge will include an indicator 9 for visual indication of the actual thickness.

'Ilhe controller for the screwdown motor which is indicated by the block 10, basically comprises electrical contactors for operating and reversing rthe direction of rotation of the screwdown motors. Such contactors may either be under manual control, as indicated by the block 11, or they may be subject to automatic control of the type to be `described hereinafter, which is schematically indicated at 12 in FIGURE 1.

Obviously, the mechanical screwdown arrangement indicated in FIGURE 1 could easily be replaced by a hydraulic system and the motors `5 could be subject to either field or armature control.

Assuming, for the moment, that contactors are used in the controller 10, lthe duration of contactor closure twill, of course, ldetermine the amount that the screw will turn per correction. If such 'duration is too long for a given set of condi-tions, the correction will be too large and there lwill be an overshoot resulting in an error in strip thick'- ness. Furthermore, some applications would time the period from the start yof the command or error signal, and which would not necessarily make the proper allowances for variations in contactor operation time, clutch and brake permissives, and other time delays. Frequently, such time delays add up to more than the actual screiw operation time and, for this reason, these variables make the time period extremely critical and ernatic for proper overall performance. The automatic control arrangement of the present invention, which will now be `described with reference to FIGURE 2, will eliminate these factors completely.

p Referring now to FIGURE 2, the armature of the screwdolwn motor 5 of FIGURE 1 is shown as 21. 'I'he armature is so connected between the supply terminals that it will rotate in either direction, depending upon which pair of contactors 1D and 2D and 1U and 2U are closed. These contactors are arranged for veither manual control by means of switches 22 and 23 used to energize relay lwindings U and D, respectively, or the same contactors may be automatically closed by their respective control contactors 24 or 25 which form a part of the automatic control system of the present invention. For direct current motors, such as indicated in this figure, it will be understood that such lconventional features as accelerating contactors, dynamic braking circuits and circuit breakers have been omitted from the drawing, as well as other protection and measuring devices.

`Connected directly across the almature terminals is a voltage-sensing, fast-acting relay 26, the normally open' contacts of which, `shown at 27, 4are utilized to actuate electronic timer circuits to be described hereinafter. It will be readily seen, therefore, that the timer circuits lare inoperative until the screwdown motor is actually energized. Because the relay 26 ris-of a fast-acting type, however,'it will operate very quickly once power is applied to the armature 21.

The on-ti-mer, which controls the time period of energizanion'of the armature 21, is Similar to a common type which is .described on' pages 168 to 17,0 of volume 19, Waveforms, Radiation Laboratory Series, McGraw- Hill Book Company, Inc., 1949, and its operation will therefore not be described in great det-ail in the present specification. Suffice to say, the timer includes a dual triode 28 and associated circuitry, such that whenever the contact 27 is closed by the fast-acting relay 26, a differentiated negative pulse is supplied to the grid 29 through a network which includes resistor 30, small differentiating capacitor 31, resistor 32, diode 33, and timing capacitor 34. Such pulse is effective to trigger the timer and at some time later, depending on the setting of variable resistor 35 and the grid potential at grid 36, the timer wilil time out and return to its normal state. This is the on-time.

In order to carry out the object of the present invention, the on-time must be a function of material characteristic as well as the deviation error derived from the thickness gauge. In other words, the screwdown screw should turn more Ias the thickness deviates more from the nominal to return the gauge more quickly to zero deviation. More sophisticated systems require additional functions such as rate, integrated error, mill acceleration and the like, as is Well known', but these will not be included in the description of the present invention. In lieu thereof, the total sum of these functions willi be hereinafter referred `to as gauge signal error and is shown in the schematic wiring diagram of FIGURE 2 as a thickness gauge 37, the reading of which is converted to an' A.C. signal by a convention-a1 converter 38 and the thus derived signal is conveniently amplified in amplifier 39. It will Ibe noted that this gauge signal is rectified by a diode 40 with the result that the signal applied to grid 36 is a function of this gauge error, thereby regulating the on-time. In other words, the greater the error, the longer the on-time.

The on-time, however, should also be a function of the material characteristics as hereinbefore mentioned. The load factor of the mill very closely follows the material characteristics. The mill load factor may be defined as the ratio of the power delivered to the strip to the speed of the stripl or approximately, the load divided by the speed. The prime mover system for -the mill is indicated in the upper left-hand portion of FIGURE 2 as a generator-motor set. In order to derive an electrical signal which is proportional to the load factor, a shunt 41 is provided in series with the armature of the prime mover, such shunt being indicated at 41. A signal taken from this shunt is amplified in an amplier 42 and compared with a speed-proportional voltage taken from a tachometer 43 driven 'by the armature of the prime mover. Conveniently, the output of the tachometer 43 is connected to an 4attenuator 44, so that the speed-proportional signal may be of .a proper value to -be compared with the load signal from -the shunt 41.

The comparison circuit for these two signals is shown as comprising a pair of identical thyratrons 45 and 46. The thyratrons `are conventionally connected through Icommon anode windings and the signal from the shunt 41 after amplification is connected to the grid of thyratron 46, while the speed-proportional signal from the tachometer 43 is connected to the grid of thyratron 45. It should be noted that the two thyratrons are not .grounded at `any point but they merely float 'between the circuits providing Ithe load-proportional signal and speedproportional signal. A /direct circuit may therefore be traced from the attenuator 44 through the adjustable attenuator arm 48, and a grid resistor of thyratron 45, and a grid resistor of thyratron 46, and back to the yamplifier 42.

The load-proportional and speed-proportional signals, therefore, are always bucking each other, and it will be apparent that -when one is higher than the other, that one or the other of the thyratrons 45 and 46 will be extinguished. When either thyratron is extinguished due to a differential in the aforementioned voltages, the current flow in the anode circuit of the on-thyratron will then `drive a shaded pole servo-motor 47 in either direction of rotation. rPhe servo 47 is connected mechanically to the `attenuator 48 and also to the adjustable arm of resistor 35 in the timing circuit of the tube 28. Thus, it will be seen that the on-time is controlled not only by the thickness gauge error -signal 'but also by the load factor which is derived by comparing the load and speed signals in the thyratron circuit. The operation of this servo-motor 47, therefore, is such that the error tends to cancel out, thus restoring both thyratrons to firing condition an'd stopping the servo.

At the end of the on-time, as determined by the time constants of the circuitry `associated with the tube 28, a pulse is delivered from lthe anode 52 of the tube 28 to the off-timer. The off-timer comprises a tube 53, which is a standard monostable multi-vibrator such as described in volume 19, Waveforms, Radiation Laboratory series, McGraw-Hill Book Company, Inc., 1949, on pages 167 to 169. The off-time is determined in part by capacitor 54, resistor 55 and the charging voltage which is fed from the output of the tachometer 43. The effect of the off-timer circuit is that the higher the speed, the shorter the off-time, which compensates for such factors as variations in the transport time of the strip from the rolls to the point of measurement. The off-timer includes in one of its anode circuits a relay 56, the purpose of which is -to disable the entire control system. As one example of how this may be effected, there is shown in series with the up and down contactors for controlling the screwdown motor, `a pair of normally closed contacts 57. It is these contacts which are open whenever the relay 56 is energized, which it will be in response to the pulse delivered to the oit-timer from the on-timer at the conclusion of the on-timer period. It will be apparent that the contacts 57 could be placed elsewhere in the circuit and stil-l be effective, as for example, they could be arranged to short out the signal from the thickness gauge which would effectively disable the up or down relays.

It is believed that the foregoing description is sufficient to enable those skilled in the art to have a complete understanding of the operation of the control circuit which, by way of summary, is as follows. Assume the mill to be running and with a thickness reduction which is the desired one, which means that no correction is being applied and the armature 21 is unenergized, Under these conditions, and further assuming a deviation in the required thickness reduction, an error signal immediately appears at the output of the converter 38. This signal is applied to the timer 28 and also, simultaneously, to the discriminator 49 which then energizes either the up 0r down relay, depending on the direction of the error signal. If the stock is heavier than required, then the down relay will be energized and vice-versa. Immediately upon energization of either the up or down relay, voltage is applied yto the armature 21 of `the screwdown motor and the fast-acting voltage .sensitive relay 26 is energized, closing contacts 27 and applying a differentiated negative pulse to the timer 28. At the conclusion of a time interval determined by the circuit constants of the timer circuit 28, a pulse will be delivered to the off-timer which is effective to energize relay 56, opening Contact 57, and restoring the circuit to normal. At the same time, the load factor of the mill is also being taken into consideration by means of the shunt 41 and the tachometer 43, the output signals from which are being continuously compared in the circuit of the thyratrons 45 and 46. As soon as either of these voltages exceeds the other, there will be rotation of servo `47 with a corresponding adjustment in the on-timer period through the potentiometer 35 and an adjustment of the attenuator 44, tending to rebalance the system. The on-time, therefore, is affected by the speed of travel of the strip through the mill, the load on the prime mover driving the mill, and also on the gauge error signal.

While many variations of the above circuitry will be apparent to those skilled in the art, a few equivalents may be listed as follows. Instead of using a servomotor 47,' completely electronic means could be used. Circuitry which will produce the ratio of load to speed is well known and, as an example, see the previously cited volume beginning at page 668. The approximate desired results may be obtained by connecting the balanced output from the tachometer and the load amplifier to an amplifier whose plate voltage will supply the charging voltage to resistor 35 in FIGURE 2. That is to say, the charging voltage would increase for the case when the tachometer voltage exceeds the load signal voltage and vice-versa. Further refinements are possible by -employing non-linear components such as varistors which would enable such characteristics'to be enhanced, as for example, by emphasizing speed effects, overload effects or increasing or decreasing the inuence of one factor at different levels.

While a preferred embodiment has been herein shown and described, applicant claims the benefit of a full range of equivalents within the scope of the appended claims.

I claim:

1. A system for automatically controlling the thickness of a strip being processed by a roll stand of the type driven by a prime mover, comprising: sensing means positioned on the finished side of the stand for sensing the stock thickness; pressure regulation means for changing the pressure |between rolls in the roll stand; means for activating said pressure regulating means in response to signals from said sensing means; and timing means responsive to at least the load on the prime mover for the stand for controlling the periods of activation of said pressure regulating means.

2. A system for automatically controlling the thickness of a strip being processed by a roll stand of the type driven by a prime mover, comprising: sensing means positioned on the iinished side of the stand for sensing the stock thickness; pressure regulation means for changing the pressure between rolls in the roll stand; means for activating said pressure regulating means in response to signals from said sensing means; and timing means responsive to speed of the stand and the load on the prime mover for the stand for controlling the periods of activation of said pressure regulating means.

3. A system as defined by claim 1 and further including means for preventing operation of said timing means prior to activation of said pressure-regulating means.

4. A system, as dened by claim l, in which said sensing means comprises an X-ray thickness gauge.

5. In combination with a roll stand including a prime mover and a reversible motor for changing pressure between the rolls; gauging means for determining the thickness of the rolled stock; means for energizing said -motor for rotation in either direction in response to deviation in stock thickness as determined by said gauging means; rst timing means for deenergizing said motor after a time interval determined by the speed of stock through the stand and the load on the prime mover; and second timing means further controlling said first timing means in response to the magnitud-e of stock thickness deviation.

6. The combination dened by claim 5, including means for deriving a signal proportional to the load on the prime mover; means for deriving a signal proportional to the speed of the prime mover; means for comparing the magnitude of said signals; and means responsive to such comparison for regulating said iirst and second timer means.

7. The combination deined by claim 6 in which the means for comparing the magnitude of said signals includes la pair of thyraltrons having said signals fed to the grids of each, and a servo controlled by said thryatrons for nulling any unbalance between said signals.

8. The combination defined by claim 1 in which said pressure regulation means includes a reversible motor connected to increase or decrease pressure on the rolls; and discriminator means connected to said sensing means and said motor for energizing said motor to increase roll pressure in response to over thick stock and to reversely energize said motor in response to under thick stock.

9. In combination with a roll stand including a pri-me mover and a motor for controlling the pressure between the rolls, an automatic stock thickness control comprising: a thickness gauge positioned on the finished side of the roll stand for providing signals proportional to deviations from a nominal thickness; means for energizing said motor in a direction to compensate for said deviation; a rst timing circuit; means connecting said motor and said first timing circuit responsive to energization of said motor to initiate the timing function; a second timing circuit; means for deriving a signal proportional to the speed of travel of the prime mover; means for deriving a signal proportional to the load on the prime mover; a servo; means for comparing the magnitude of said speed-proportional and load-proportional signals, rsaid means being connected to `drive said serrvo whenever said signals differ, said servo being connected to rebalance said signals in response to being driven by a difference therebetween; means controlling said rst and second timing circuits in response to rotation of said servo; and means for controlling the time of energization of said motor in response to said timing circuits.

References Cited in the file of this patent UNITED STATES PATENTS 2,264,095 Mohler Nov. 25, 1941 2,708,254 Macaulay etal. May 10, 1955 2,767,604 Whalen Oct. 23, 1956 2,972,269 Wallace et al. =Feb. 21, 1961 OTHER REFERENCES Rolling Slabs Into Strip Steel, Control Engineering, September 1956, pp. 116-117. 

1. A SYSTEM FOR AUTOMATICALLY CONTROLLING THE THICKNESS OF A STRIP BEING PROCESSED BY A ROLL STAND OF THE TYPE DRIVEN BY A PRIME MOVER, COMPRISING: SENSING MEANS POSITIONED ON THE FINISHED SIDE OF THE STAND FOR SENSING THE STOCK THICKNESS; PRESSURE REGULATION MEANS FOR CHANGING THE PRESSURE BETWEEN ROLLS IN THE ROLL STAND; MEANS FOR ACTIVATING SAID PRESSURE REGULATING MEANS IN RESPONSE TO SIGNALS FROM SAID SENSING MEANS; AND TIMING MEANS RESPONSIVE TO AT LEAST THE LOAD ON THE PRIME MOVER FOR THE STAND FOR CONTROLLING THE PERIODS OF ACTIVATION OF SAID PRESSURE REGULATING MEANS. 